Absolutely they can win majors at 18 or 19 and it is not uncommon. It does not necessarily mean that that is when they are in their prime.There's a pretty long list of women tennis players winning grand slams in their teens (as young as 16): Hingis, Seles, Austin, Sharapova, etc. https://en.wikipedia.org/wiki/List_...champions_in_Open_Era_with_age_of_first_title
It appears that the top women players start winning in their teens. It's certainly not a rarity. It seems closer to being the rule.